In this case 5 is the number of retries attempted to read the data before giving up, /dev/sdf is the path to the device that you found via fdisk -l /tank/Backup/image is the image file that will be written. /tank/Backup/logfile is a logfile that is kept of the recovery.

If the backup is interrupted or you want to run it for another pass with hopes of recovering more information use a command like this: